Technical Field
The present invention relates to optical switches and, more particularly, to optical switches used to control directions in which optical signals are transmitted or received, such as in light direction and ranging (LiDAR) and laser communication systems.
Background Art
Many optical system need to emit or receive a (visible or invisible) collimated optical beam in a direction that can be controlled with high precision. For example, such optical beams are used in light direction and ranging (LiDAR) systems, and often these beams need to be steered or swept to locate or track a target. Similarly, laser communications systems sometimes need to steer an optical beam, such as to initially establish a line-of-sight communications channel between two terminals or if one or both of the terminals moves.
Prior art LiDAR and laser communications terminals use telescopes and either point the entire telescope using a gimbal or place a moveable steering mirror in front of the telescope and use the mirror to redirect the beam. However, this approach requires large and bulky moving systems, with attendant disadvantages in terms of size, mass, power and reliability.
Other conventional methods of beam steering involve optical phased arrays, in which a large number of antennas are arrayed closely together and operated coherently, i.e., the phases of the individual emitters are carefully controlled to make the entire array operate in unison. Signals in the near field constructively and destructively interfere to create nulls and reinforced signals in desired directions. However, phased arrays require large numbers of emitters and associated optical phase adjusters.
A nominal optical phased array has emitters disposed at half-wavelength spacings, i.e. apx. 0.5 μm. For applications, such as long-range laser communication, the required total aperture size might be on the order of 5 cm. Thus, one would need an array of 104×104 emitters and phase shifters. As currently demonstrated, phase shifters requires apx. 1 mW of power to operate. Thus, the total power consumption of such an array might approach 105 W, an impractically large amount of power.
U.S. Provisional Patent Application No. 62/498,158, filed Dec. 16, 2016, titled “All-solid state optical transmit/receive terminal,” ('158 patent application) the entire contents of which are incorporated by reference herein for all purposes, is assigned to the assignee of the present application. The '158 patent application discloses an optical transmit and receive terminal that includes an N×1 optical switch. The N×1 optical switch includes an “H-tree” arrangement of optical waveguides, rooted at a common input/output port. A binary optical switch is disposed at each junction of two optical waveguides of the H-tree. Settings of the binary optical switches determine a route for an optical signal traveling through the H-tree. Each binary optical switch may be implemented with a Mach-Zender interferometer, ring resonator or microelectromechanical systems (MEMS) device, such as a MEMS mirror. However, limitations on the densities at which the binary optical switches may be fabricated limit spatial resolution of the transmit/receive terminal, i.e., density of possible transmit or receive beam positions.